A variety of adhesive sheet materials are currently used in the manufacture of adhesive labels, adhesive tapes, and double-sided adhesives. Adhesive sheet materials may be formed by providing a layer of adhesive on the surface of a sheet substrate such as paper, plastic film or metal foil. The layer of adhesive can be bonded to the surface of an article without the need to moisten the adhesive. Owing to the convenience of their use, there is a great and growing demand for adhesive sheet materials.
The sticky adhesive surface of an adhesive sheet material, adhesive patch, adhesive pad, or other adhesive product is often provided with a peelable or releasable protective sheet known as a release backing, a release sheet, a release layer or a release liner. The backing temporarily protects the sticky adhesive surface of the adhesive product from contamination by dust, debris, moisture and other contaminants until the label is ready for use. Generally, the peelable release backing is peeled from the adhesive surface immediately before use of the adhesive product.
The performance of the temporary protective backing is largely dependent on the combination of the adhesive which forms the adhesive layer of the adhesive product, and the release or anti-stick agent which imparts releasability to the surface of the peelable release backing. The release backing, once bonded to the sticky adhesive surface, must not spontaneously come off without a peeling force, but it must be readily removed with a relatively small peeling force, and the backing should not cause a decrease in the residual sticking power of the adhesive. It is also important that the backing releases easily enough from the adhesive product that the product does not tear when separated from the backing, and so that the adhesive layer of the product remains intact during separation.
With double-sided adhesive tapes, patches, and products it is desirable to have a peelable backing of a first release force on a first surface of the adhesive, and a peelable backing of a greater release force on the opposite second surface of the adhesive. Thus, the first adhesive surface can be adhered to an article while the second adhesive surface remains protected by the second backing. The second adhesive surface can remain protected until it is desired to expose the second adhesive surface for contact with a second article or another surface. For a rolled double-sided tape, a single protective sheet may be used having opposing release backing surfaces of different release characteristics, as opposed to providing two separate backings. A system of variable release backings or backing surfaces is therefore necessary to provide backing surfaces of different specified release force.
Silicone coated release substrates are known, particularly in applications where the substrate is paper. Silicone coated release substrates are most frequently produced by coating the substrate with a solvent or solventless silicone-based resin composition. As the silicone resin composition dries, it adheres to the substrate. A silicone release coating generally requires an extremely low release force to peel an adhesive therefrom. To date, a silicone release coating system has not been provided which enables a series of coatings of various release forces achieved by varying component amounts in the system.
U.S. Pat. No. 4,423,095 to Blizzard discloses a water-resistant adhesive release coating for a substrate such as paper. The coating composition comprises a liquid silicone resin and an organic polymer wherein the properties of the coating are not merely fractionally additive, based on weight, of the properties of the individual components, but are substantially undiminished from the respective properties of the neat components.
A need exists for an adhesive release composition which has release properties that vary in proportion to component weight ratios in the composition. A need also exists for a series of release coating compositions which can provide a series of release surfaces, for example, for double-sided tape, wherein each member of the series exhibits a predictable and repeatable release force and has a release force that is substantially linearly related to the compositional make-up of each member of the series. A need also exists for a release coating composition which can provide a release surface of uniform, repeatable, consistent and specified releasibility.
In view of the ever-growing need for recyclable and reclaimable industrial products, it would be advantageous to provide a release backing material that is reclaimable, particularly a reclaimable film substrate. Significant amounts of scrap film are generated during film-forming processes. Scrap film is normally comminuted, melted, extruded into the form of pellets, mixed with fresh virgin polymer, re-melted and re-fed to the film forming extruder. Temperatures of about 290.degree. C. to about 300.degree. C. may be encountered during reclamation processing of polyester reclaim film. Thus, for reclaimability, the coating on the film must be stable at these processing temperatures and must not impart an undesirable yellow discoloration or an undesired amount of impurities or gels to a finished product containing significant amounts of the reclaimed film. Preferably the coating can be effective at very low coat weights to minimize impurification of the ultimate product containing the reclaimed film.
Components that are known to decrease the reclaimability of many polymeric products include agents that depolymerize or decompose at reclamation temperatures, causing volatile or gaseous odoriferous by-products which deleteriously affect the reclaimed product or pose safety problems. Some coating compositions, for example, polyvinyl alcohol coatings, char at polyester reclamation temperatures and cause undesirable black specks and impurities or gels in a reclaimed polyester product. Crosslinking agents have been used in film coating compositions to enhance the adhesive qualities of a film coating and the durability of film coatings. However, as disclosed in U.S. Pat. No. 4,571,363 to Culbertson, et al., crosslinking agents may give rise to undesirable yellowing or discoloration when subjected to reclamation temperatures.
It has been found that the use of substantial amounts of coating compositions, cross-linking agents and/or nitrogen-containing antistatic agents in polymeric film coatings tends to result in yellowing, discoloration and foul odors when subjecting film sheets or scraps containing the agents to reclamation temperatures. It has been found that the use of substantial amounts of coating compositions, for example, more than about 1000 ppm based on the total weight of the coated film, tends to form undesired impurities or gels during reclamation. Thus, there is a need for a reclaimable release backing including a release coating which requires less than 1000 ppm coating composition, based on the weight of the coated film, to be effective and which does not substantially adversely affect reclaimability of the coated film substrate.
To avoid the production of coated scrap which may not be reclaimable in the production of oriented films, scrap may be removed along the longitudinal edges of the film where the film is held and stretched by tenters, prior to coating. However, as opposed to in-line coating techniques, removing scrap prior to coating requires additional processing because the film must be made and cut at one location, rolled, transported to another location, unrolled and coated off-line. In-line coating of film saves time and is more economical.
In-line coating techniques have been employed in the manufacture of coated polymeric films and provide efficient processes for forming uniform and thin films and coatings. It is desirable to provide an in-line coating method for forming a controlled release coating on the surface of a thin polymeric film, for example, on a polyester film. Polyester film can be used as a substrate for a release backing and can generally be made by extruding an amorphous melt of polyester onto a polished cooled casting drum to solidify the melt into sheet form. The sheet then undergoes a heating and stretching operation that orients the polymer chains making up the film, thus giving the film strength, stability and other desirable physical properties. After the film has been stretched in one or both direction(s), the film is generally heat-set at a temperature higher than the temperature at which it is stretched. Heat setting serves to lock-in the physical properties of the stretched polyester film. Manufacturing processes for producing polyester films are well known.
A coating composition for in-line application to a film surface should generally be quickly settable or curable for compatibility with film production rates. Additionally, the coating composition should be capable of being drawn with the substrate without undergoing deterioration in strength and transparency.
There is a need for a release coating composition which can be formed in-line during a polymeric film manufacturing process and which requires no extraneous heating step to dry or cure the coating composition, but which instead takes advantage of the heat necessary for film orientation in a drying or curing step.
The present invention provides a controlled release backing having a release surface of repeatable, consistent, homogeneous and specified releasibility. The present invention also provides a system of compositions which can provide a progressive series of release surfaces having different release forces wherein each member of the series exhibits a repeatable and consistent relationship between its compositional make-up and the release force.
The present invention provides a process for making a controlled release backing in-line during a polymeric film manufacturing process and which requires no extraneous heating step to dry or cure the release coating composition which is applied in-line to the film substrate.
The present invention also provides a reclaimable controlled release backing comprising a coating composition which does not substantially adversely affect reclaimability of a polyester film substrate and which does not undergo substantial gel or impurity formation, yellowing, discoloration or odor formation upon being subjected to polyester film reclaiming temperatures. The present invention provides a coated polyester controlled release substrate comprising a release surface made of a composition which, at an effective coat weight, enables useful reclamation of the polyester substrate without causing substantial impurity formation, yellowing and odor.